EP0796367B1 - Production of prehydrolyzed pulp - Google Patents

Production of prehydrolyzed pulp Download PDF

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Publication number
EP0796367B1
EP0796367B1 EP95905669A EP95905669A EP0796367B1 EP 0796367 B1 EP0796367 B1 EP 0796367B1 EP 95905669 A EP95905669 A EP 95905669A EP 95905669 A EP95905669 A EP 95905669A EP 0796367 B1 EP0796367 B1 EP 0796367B1
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EP
European Patent Office
Prior art keywords
liquor
cooking
digester
hydrolysate
neutralized
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP95905669A
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German (de)
English (en)
French (fr)
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EP0796367A1 (en
Inventor
Panu Tikka
Kari Kovasin
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Metso Paper Pori Oy
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Sunds Defibrator Pori Oy
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    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C3/00—Pulping cellulose-containing materials
    • D21C3/02—Pulping cellulose-containing materials with inorganic bases or alkaline reacting compounds, e.g. sulfate processes
    • D21C3/022—Pulping cellulose-containing materials with inorganic bases or alkaline reacting compounds, e.g. sulfate processes in presence of S-containing compounds
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C1/00—Pretreatment of the finely-divided materials before digesting
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C3/00—Pulping cellulose-containing materials
    • D21C3/22—Other features of pulping processes
    • D21C3/26—Multistage processes

Definitions

  • the present invention relates to a process for the production of special pulp from material lignocellulose-containing.
  • hemicelluloses are hydrolysed into hydrolysate, and lignin is dissolved by a kraft cooking method for liberating cellulose fibers.
  • the produced pulp has a high content of alpha cellulose and can be used e.g. as dissolving pulp.
  • a separate prehydrolysis step permits the desired adjustment of the hydrolysis of hemicelluloses by varying the hydrolysis conditions.
  • the prehydrolysis is carried out either as a water or steam phase prehydrolysis or in the presence of a catalyst.
  • organic acids liberated from wood in the process perform a major part of the hydrolysis, whereas in the latter, small amounts of mineral acid or sulfur dioxide are added to "assist" prehydrolysis.
  • the delignification step has been a conventional kraft cooking method, where white liquor has been added to the digester and the cooking has been carried out as a single step after removing some or none of the prehydrolysate.
  • One of the drawbacks of this process is e.g., that the neutralized hydrolysate (free hydrolysate left in the digester, as well as immobilized hydrolysate inside the chips) causes consumption of cooking chemicals and loading of the digester.
  • US-A 4 436 586 discloses a mild acid prehydrolysis step followed by a mild caustic pre-extraction step prior to kraft cooking, in a separate vessel of a continuous cooking process. Specific cooling steps and equipment are required because of the relatively low temperatures in the pre-extraction zone. At the bottom of the separate prehydrolysis and pre-extraction vessel, white liquor is introduced to raise the temperature of the material close to the cooking temperature for introduction into the digester.
  • US-A 4 436 586 requires a low temperature, low alkali concentration pre-extraction step carried out at a temperature of about 60-90 °C.
  • the chemical processing of the chips is thus essentially interrupted.
  • the prehydrolyzed material is transferred to another reactor for delignification.
  • the possibilities to optimize processing times for the separate steps is very limited.
  • the object of the present invention is to provide an improved prehydrolysis-kraft process for the preparation of pulp with a high content of alpha cellulose. This is accomplished by means defined in the independent claims. Some preferable embodiments of the invention are defined in the dependent claims.
  • the process comprises prehydrolyzing the cellulosic material, neutralizing the hydrolyzed material with alkaline liquor as defined in the present claim 1, removing the neutralized hydrolysate from the digester, and delignifying the prehydrolyzed and neutralized material with alkaline cooking liquor containing sodium hydroxide and sodium sulfide.
  • the present invention offers e.g. following advantages:
  • Material to be used in the process is suitably softwood or hardwood, preferably hardwood such as, e.g., eucalyptus species, beech, or birch.
  • Suitable neutralizing agents contain caustic soda.
  • the prefered agent is alkaline kraft cooking liquor, i.e., white liquor.
  • a suitable neutralization time is 10 - 40 min, preferably 20 - 30 min, which is enough to get the digester content mixed.
  • the neutralization temperature is 140 - 160 °C. and the neutralization alkali charge is 5 - 20 % active alkali calculated as Na 2 O equivalents on dry wood. This results in a neutralization residual alkali concentration of 1 - 20 g of effective NaOH/liter, depending on the wood species and charge.
  • the removal of neutralized hydrolysate is suitably carried out by displacement with hot black liquor originating from a previous cook.
  • the hot displaced black liquor preferably has a residual alkali concentration of 10 - 25 g of effective NaOH/liter, a pH 12.5 - 13.5, and a temperature between 150 - 180 °C.
  • the hot black liquor reacts with the wood material, whereby the residual alkali concentration of the hot black liquor is consumed, and pH is decreased.
  • the displacement with hot displaced black liquor suitably provides a reaction time of 10 - 30 minutes. The reaction facilitates the delignification with fresh alkaline cooking liquor in the cooking step.
  • the displacement is continued with fresh alkaline cooking liquor (white liquor) introducing the cooking alkali charge, which preferably is 5 - 15 % active alkali calculated as Na 2 O equivalents on dry wood.
  • the portion of sodium sulfide of the white liquor active alkali (the sulfidity) is suitably 15 - 45 % calculated as Na 2 O equivalents.
  • the preferable temperature of the alkaline cooking liquor is 150 - 180 °C.
  • the cooking phase is suitably carried out by circulating the cooking liquor 10 - 120 min and adjusting the desired cooking temperature by means of high pressure steam, preferably by direct steam injection to the circulating cooking liquor.
  • a suitable cooking temperature is 150 - 180 °C, preferably 150 - 165 °C for hardwoods, and 155 - 170 °C for softwoods.
  • the cooking step is preferably terminated by displacing the hot black liquor by means of using cooler liquor, preferably a wash filtrate having, e.g., a temperature of 60-90 °C.
  • the hot displaced black liquor which is rich in dissolved solids and sulfur compounds is preferably recovered for re-use, and the heat of the rest of the displaced hot liquor is recovered by heat exchange.
  • the pulp is suitably discharged from the digester by pumping.
  • the displacements are preferably carried out from the bottom to the top of the reactor.
  • prehydrolysis-kraft pulp can be delignified to lower residual lignin concentration while maintaining excellent pulp quality in terms of pulp viscosity and alpha cellulose purity, for such end uses as dissolving and other special pulps. Simultaneously the energy economy of the process can be improved.
  • Figure 1 is a schematic representation of the tanks and liquor transfer sequences according to a process in accordance with the present invention.
  • Suitable prehydrolyzing agents include, e.g., water as circulating liquid or in the steam phase, aqueous solutions of mineral acids such as sulfuric or hydrochloric acid, sulfur dioxide and acid bisulfite cooking liquor.
  • Preferable prehydrolyzing agents for softwoods include water, and for hardwoods water, sulfuric acid or sulfur dioxide.
  • a suitable prehydrolyzing temperature is 100 - 160 °C for softwoods and 120 - 180 °C for hardwoods.
  • a suitable hydrolyzing time is 10 to 200 min, preferably 20 - 120 minutes.
  • part of the hydrolysate can be recovered before the neutralization step, and can be used, for example, for producing ethanol.
  • the prehydrolysis step deviates from prior art prehydrolysis-kraft processes.
  • the prehydrolysis is followed by a new, individual step; the neutralization step.
  • the primary purpose of this step is to neutralize the hydrolysate left in the digester. There is hydrolysate both in the free liquid outside the chips and also trapped and immobilized inside the chips.
  • fresh hot white liquor Al is pumped from tank A into the digester so as to displace the hydrolysate from outside the chips.
  • the neutralization is completed by circulating the liquor in the digester, and thus mixing the content.
  • contents of the digester are prepared for later delignification, to be carried out by alkaline kraft cooking.
  • Neutralization is achieved by selecting an appropriate neutralizing alkali charge which results in clearly alkaline neutralization end point.
  • the residual alkali concentration is preferably 5 - 15 g effective NaOH/liter. This levels out fluctuations in terms of improper alkali charge and pulp quality due to fluctuating consumption of the single alkali charge by the neutralization.
  • the neutralization step also serves as an alkaline hemicellulose dissolving step.
  • the strong alkali and the high temperature directly dissolve and, on the other hand, degrade hemicelluloses by the so called end-wise peeling reaction.
  • the pulp is thus further purified, which leads to higher pulp viscosity and higher alpha cellulose content.
  • the neutralization step also becomes, in part, an alkaline extraction stage prior to the cooking step. Therefore the liquor-to-wood ratio in this step is preferably relatively low, between about 2.5 - 3.5.
  • Mn, Fe, Cu, and Co dissolved in the acidic prehydrolysis step, are removed from the digester, and thus the disadvantageous metal ion content of the cooked pulp is lowered. This facilitates oxidative bleaching of the pulp with oxygen, peroxide and ozone.
  • the hot black liquor flow to the digester is continued by flow B2 from the tank B, turning the entire contents of the digester to be submerged in the hot black liquor, and the temperature of the digester content comes close to the temperature of the hot black liquor which, in turn, is close to the cooking temperature.
  • the displaced liquor C2 flows to the hot displaced liquor tank C.
  • the sulfide rich hot black liquor reacts with the wood material and greatly facilitates the delignification with fresh alkaline cooking liquor in the cooking step.
  • the hot black liquor reaction step is carried out for a period of from 10 - 30 minutes, whereby the residual alkali concentration of the hot black liquor, which is preferably 10 - 25 g effective NaOH/l, is consumed to preferably 1 - 10 g effective NaOH/l.
  • the pH of the hot black liquor preferably 12.5 - 13.5, is decreased to between about 9.5 - 11.5 in the liquor inside the chips and between about 11.5 - 12.5 in the free liquor outside the chips.
  • hot white liquor A2 from the tank A is pumped to the digester displacing a corresponding volume C3 of the hot black liquor based cooking liquor to the hot displaced liquor tank C.
  • the hot liquor from this tank is led through heat-exchangers to an atmospheric evaporation liquor tank E which serves as a buffer tank discharging the liquor to the evaporation plant and recovery of cooking chemicals.
  • Hot liquor from the tank C is used to heat white liquor to be pumped to the tank A, and to prepare hot water.
  • the hot white liquor addition A2 starts the kraft cooking step, i.e. the final delignification. Due to the high temperature of the hot black liquor, the starting temperature after the white liquor addition A2 is high, close to the desired cooking temperature. Therefore the heating-up step is in fact a temperature adjustment step, where the need to heat up is preferably only 1 - 10 °C. This can be achieved by simple direct high pressure steam flow to the circulation pipe line, thus avoiding expensive heat-exchangers.
  • the cooking step is very short.
  • the degree of reaction of the digestion conditions which is required i.e., reaction temperature and time
  • H-factor Prior art prehydrolysis kraft cooking with hardwoods requires 800 - 1200 H-factor units to complete the delignification, whereas the present prehydrolysis-displacement kraft cooking process needs only about 400 H-factor units to reach the same and even higher delignification degree. If the same cooking temperature would be used, this means cutting the cooking time to 35 - 50 % of that of the prior art conventional prehydrolysis-kraft cooking time.
  • the consequence of greatly reduced need for cooking time is that the cooking step can be made very mild providing improved pulp quality.
  • the cooking advantage of H-factor 400 instead of the conventional H-factor requirement of 1000, is converted to lower cooking temperature, it is possible to use the cooking temperature 159 °C in stead of the conventional 170 °C.
  • the high unbleached pulp intrinsic viscosity is very valuable, since the new more and more compulsory total chlorine free oxidative bleaching sequences compromise the viscosity much more severely than the conventional and more selective chlorine chemicals based bleaching.
  • the present invention enables the production of high quality prehydrolysis-kraft pulp by using totally chlorine free bleaching sequences.
  • the cooking step is terminated by the displacement of the cooking liquor with cool, preferably 60 - 90 °C, displacement liquor from the tank D.
  • This liquor is preferably filtrate from the pulp wash plant.
  • the first portion B of the displaced black liquor consists of pure black liquor and covers the dry solids rich portion of the displaced liquor.
  • the volume of this displaced portion varies depending on the wood density and digester filling degree, but is usually referably close to the free liquor volume of the digester, typically between about 60 - 70 % of the digester total volume.
  • the separation is done according to a precalculated volume or by monitoring the dissolved solids concentration of the displaced liquor.
  • the displaced liquor which is still hot but has been diluted by the displacement liquor is recovered to the hot displaced liquor, tank C which sends its content through the heat exchange only to the evaporation liquor tank E and out of the cooking process.
  • the result is that only the dissolved solids and sulfur chemicals rich hot black liquor B is re-used in the displacement of the neutralized hydrolysate and in the subsequent hot black liquor treatment.
  • the digester is discharged after the terminal displacement step by pumping the content out.
  • Example 1 (Comparative examples). Production of prehydrolysis-kraft pulp by means of a conventional prehydrolysis-kraft-batch process from Eucalyptus Grandis chips
  • Chips were metered into a chip basket positioned in a 35-liter forced circulation digester.
  • the cover of the digester was closed and the prehydrolysis was carried out according to the temperature program by introducing direct high pressure steam into the digester.
  • the cooking liquor charge was pumped into the digester and the digester circulation started.
  • the cooking was carried out according to the cooking temperature program by heating the digester circulation be mens of steam.
  • the cooking liquor was rapidly cooled and the spent liquor discharged.
  • the pulp was washed in the digester and then discharged from the cooking basket to disintegration for 3 minutes. After the disintegration, the pulp was dewatered and the total yield determined. Then the pulp was screened on a 0.25 mm slotted screen.
  • Shives were measured and the accept fraction was dewatered and analyzed.
  • the conditions were: Prehydrolysis step Wood amount, g of abs. dry chips 2000 Prehydrolyzing agent direct steam Temperature rising, min 60 Prehydrolysis temperature, °C 170 Prehydrolysis time, min 25 Kraft cooking step Active alkali charge, % on wood as Na 2 O 18 White liquor sulfidity, % 36 Temperature rising time, min 60 Temperature, °C 170 Cooking time, min 60 Cooking H-factor 1100 Yield, % on wood 38.4 Shive content, % on wood 0.1 Kappa number 10.0 Viscosity SCAN, dm 3 /kg 905 Alkali solubility S5, % 2.4 Brightness, % ISO 34.0
  • Example 2 (Comparative examples). Production of prehydrolysis-kraft pulp by means of a conventional prehydrolysis-kraft-batch process from Eucalyptus Grandis chips
  • Example 3 Production of prehydrolysis-kraft pulp by means of a batch process in accordance with the invention from Eucalyptus Grandis chips.
  • Chips were metered into a chip basket positioned in a 35-liter forced circulation digester.
  • the cover of the digester was closed and the prehydrolysis was carried out according to the temperature program by introducing direct high pressure steam into the digester.
  • neutralization white liquor was pumped into the digester and the circulation was started.
  • hot black liquor was pumped into the digester bottom.
  • the pumping first filled the digester up and then continued as displacement ousting liquor from the top of the digester.
  • the hot black liquor pumping was stopped after the desired volume was pumped in.
  • the digester circulation was started again, and the desired temperature was reached.
  • the desired viscosity was 1050 - 1100 dm 3 /kg, and it could be achieved by delignifying less, in other words cooking to higher kappa number, typically to 11 - 13 for Eucalyptus grandis.
  • This kind of conventional Eucalyptus prehydrolysis-cook resulted in about 40 % yield.
  • Example 1 demonstrates the results from a conventional prehydrolysis-kraft batch cook, where the delignification has been extended to kappa number 10. As can be seen, the pulp viscosity is too low. In addition, the pulp yield is quite low increasing the manufacturing costs.
  • Example 2 shows the result, when the conventional prehydrolysis-kraft batch cook has been changed to produce the required pulp viscosity 1200 dm 3 /kg by adding alkali charge and cutting down cooking time and temperature. As a result, the kappa number stays much too high for the above requirements.
  • Example 3 demonstrates the result, when the process is carried out according to the present invention.
  • the required viscosity 1200 dm3/kg has been reached while achieving the delignification down to the kappa number 9.1, and the pulp yield close to the conventional 40 %, which has been the case at about 50 % higher kappa number level of 14.
  • the alkali solubility percentage was well acceptable and fairly constant in all examples.
  • Another proof for better bleachability of the pulp produced according to the present invention is the brightness of the unbleached pulp.
  • the conventional prehydrolysis pulp in examples 1 and 2 show the brightness 32 - 34 % ISO, whereas the pulp in example 3 has the brightness 40 % ISO; i.e., a significant 20 % improvement in brightness and bleachability.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Paper (AREA)
  • Polysaccharides And Polysaccharide Derivatives (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
  • Compounds Of Unknown Constitution (AREA)
  • Saccharide Compounds (AREA)
EP95905669A 1994-01-24 1995-01-23 Production of prehydrolyzed pulp Expired - Lifetime EP0796367B1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
FI940332 1994-01-24
FI940332A FI103898B (fi) 1994-01-24 1994-01-24 Menetelmä prehydrolysoidun sellun ja/tai sellumassan tuottamiseksi
PCT/FI1995/000028 WO1995020065A1 (en) 1994-01-24 1995-01-23 Production of prehydrolyzed pulp

Publications (2)

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EP0796367A1 EP0796367A1 (en) 1997-09-24
EP0796367B1 true EP0796367B1 (en) 2001-06-13

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EP95905669A Expired - Lifetime EP0796367B1 (en) 1994-01-24 1995-01-23 Production of prehydrolyzed pulp

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EP (1) EP0796367B1 (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)
JP (1) JP2984798B2 (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)
CN (1) CN1102973C (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)
AT (1) ATE202166T1 (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)
AU (1) AU688447B2 (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)
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FI (1) FI103898B (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)
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CA2179914C (en) 2005-09-20
DE69521336D1 (de) 2001-07-19
JP2984798B2 (ja) 1999-11-29
DE69521336T2 (de) 2002-04-25
AU1419495A (en) 1995-08-08
EP0796367A1 (en) 1997-09-24
JPH09507697A (ja) 1997-08-05
FI103898B1 (fi) 1999-10-15
BR9402395A (pt) 1995-10-17
AU688447B2 (en) 1998-03-12
FI103898B (fi) 1999-10-15
ZA95388B (en) 1995-09-26
WO1995020065A1 (en) 1995-07-27
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MY114661A (en) 2002-12-31
FI940332L (fi) 1995-07-25
US5589033A (en) 1996-12-31
PH31329A (en) 1998-07-06
NO963072L (no) 1996-07-23
TW317585B (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html) 1997-10-11
ATE202166T1 (de) 2001-06-15
FI940332A0 (fi) 1994-01-24
ES2159311T3 (es) 2001-10-01

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